Much has been written in the last decade about the use of special earmold constructions to change the frequency response of a hearing aid by modifying the sound channel. These modifications range from simple changes in the diameter of the sound channel at appropriate locations (Killion, M.C. "Earmold options for wideband hearing aids", J. SPEECH AND HEARING DISORDERS 46, 1981, p. 10) to more complicated designs using cavity resonances either in the main sound channel (Goldberg, H. "An extended range universal earmold", HEARING AID JOURNAL, Mar 1978, p. 10) or in the vent channel (Macrae, J. "Venting without feedback-further development of the high-cut cavity vent", HEARING INSTRUMENTS, 33, Apr. 1982 p. 12).
Special earhook designs can also be used to modify the frequency response of a hearing aid. Existing designs include the "high pass" earhook with a small hole near the tip (Berland, O. "No-mold fitting of hearing aids", in EARMOLDS AND ASSOCIATED PROBLEMS, S. Dalsgaard, Ed., SCANDINAVIAN AUDIOLOGY Suppl. 5, 1975, p. 188) and an "E-HOOK" with a small plastic insert (Bergenstoff, H. "Recent Development in hearing aid design and fitting techniques", Danavox Report 81-09-28, Danavox A/S Copenhagen, 1981, p. 6 and FIG. 11).
Another prior development relates to a lowfrequency emphasis, open-canal hearing aid including a specially constructed hearing aid unit cooperating with an earhook which contains two filter chambers (Killion, M. C., Berlin, C.I. and Hood, L. "A low frequency emphasis open canal hearing aid." HEARING INSTRUMENTS 35, Aug. 1984, 30-34.).
An additional prior disclosure relates to the use of a notch filter within a specially designed earmold. (Macrae, J. "Acoustic modifications for better hearing aid fittings." HEARING INSTRUMENTS, 34, Dec. 8,11, 1983). That earmold was designed to compensate for a type of hearing loss that "may occur too rarely to justify the production of a model of hearing aid designed especially for it".
Still another prior disclosure relates to a specially designed earmold system. (Lybarger, S. "A special purpose dual-diameter earmold system." Presented at American Hearing Research Foundation Workshop, Chicago, June 1980.) That system was described as being "especially effective for open canal fittings on persons whose hearing remains fairly normal up to 1500 or 2000 Hz and then drops off markedly."
Such prior constructions have been used advantageously but have been expensive to manufacture, have not always met the needs of wearers and have had problems and disadvantages not previously recognized.